Single-phase inverter circuit to condition and transform direct current electric power into alternating current electric power

ABSTRACT

The invention presents a structure for the conversion of direct current electric power into alternating current electric power, characterised in that it is simple, highly efficient and minimises the problem of electromagnetic compatibility. The circuit includes, in its first preferred embodiment, six switching elements governed by a command unit, four switches forming an H-bridge (T 1,  T 2,  T 3,  T 4 ) and two auxiliary switches (T 5 D, T 6 D), and two auxiliary diodes (Daux 1  and Daux 2 ). The elements of the H-bridge switch at grid frequency, whereas T 5 D and T 6 D switch at high frequency by means of pulse width modulation (PWM), or other appropriate modulation techniques. The voltage of these auxiliary switching elements (T 5 D, T 6 D) is limited topologically to half the direct current input voltage (Vin), thereby reducing switching losses and resulting in a high performance converter.

OBJECT OF THE INVENTION

The present invention has its main field of application in the industrydedicated to the design of electronic devices and, more particularly, tothose conceived within the sector of photovoltaic solar energy powersystems.

The object of the invention is to provide a dc/ac conversion structurespecially designed for photovoltaic systems connected to the electricgrid without a transformer, characterized in being simple, highlyefficient and minimizing the problems of electromagnetic compatibility.

BACKGROUND OF THE INVENTION

Photovoltaic systems connected to the grid are today enjoying wideacceptance in our society, and are now being used with increasingfrequency. Commonly, they involve installations formed by a group ofsolar panels and an electronic converter, called an inverter whichconditions the energy produced by the panels and injects it into theelectric grid.

In most cases, the converters for these installations are privatelow-power single-phase circuits, wherein the objective sought is tomaximize the economic return obtained by selling the energy produced toelectricity companies. For this reason, the inverters required must becheap, reliable and highly efficient.

Usually a low-frequency transformer is included in the conversion stagein photovoltaic installations connected to the grid. This transformerguarantees galvanic isolation between the installation and the grid andreduces electromagnetic emissions. The main drawback of thelow-frequency output transformer is its considerable size and weight, aswell as substantially increasing the price of the conversion stage and,therefore, making the photovoltaic installation as a whole moreexpensive.

The evolution of technology means that today it is possible to dispensewith this transformer without reducing the properties of the system asregards personal safety and integration in the grid, thereby questioningthe need for its use. In the near future, the obligation to use anoutput transformer is expected to gradually disappear.

If the transformer is removed, the galvanic isolation between thephotovoltaic system and the grid is lost, which impairs system behaviourregarding Electromagnetic Compatibility (EMC). In this event, it isdesirable to use conversion topologies that minimize EMC problems.

The use of a transformer in photovoltaic systems has allowed an H-bridgewith unipolar modulation to be employed as the conversion structure,which structure has demonstrated the best efficiency versus complexityrelationship. However, this structure behaves poorly from the point ofview of EMC. FIG. 1 shows an H-bridge. This structure comprises twoparallel branches, each with its own pair of switches or switchingelements in series (T1, T2 and T3, T4), usually transistors, with diodesin anti-parallel (D1, D2, D3, D4).

An option to improve inverter behaviour regarding EMC, is to use bipolarmodulation. In this modulation, the T1-T4 and T2-T3 switch pairs switchalternately, obtaining voltages at the output points of the H-bridgehaving the value of the input voltage with positive and negative sign(+Vin or −Vin). However, bipolar modulation has two disadvantages withrespect to unipolar modulation. On one hand, the current ripple in thecoil with bipolar modulation is greater. On the other hand, to obtainthe same current ripple frequency in the coil in bipolar modulation, itis necessary to switch at twice the frequency, which means it has twicethe switching loss. This, together with the fact that the semiconductorshave to withstand all the input voltage, implies a decrease in theefficiency obtained with this structure.

In the resolution of the deficiencies in the structures of theaforementioned inverters, it is necessary to cite the proposal ofEuropean patent application EP1369985. The inverter circuit described inthe aforementioned document consists of an H-bridge switching at highfrequency with bipolar modulation, to which a third branch is added onthe alternating side, between the output points (A, B) of the fullbridge inverter which is switching at the grid frequency, as shownschematically in FIG. 2. This structure, which includes six transistors,improves the behaviour and global efficiency of the photovoltaicconverter with respect to the H-bridge with bipolar modulation,according to the operating mode explained in EP1369985.

This converter described in EP1369985 has two advantages with respect tothe H-bridge with bipolar modulation: one, the switching of thetransistors of the H-bridge is carried out with half the input voltage,which reduces the switching losses of the converter; two, the maximumcurrent ripple in the coil is half that in the bipolar H-bridge, whichallows a smaller coil to be used. However, although the transistorsswitch with half the input voltage, in the cutoff state they support allthe input voltage (Vin) whereby all of them, those in the H-bridge plusthose of the additional branch on the alternating side (T5A-T6A), haveto be sized for said voltage. Since switching losses increase with thevoltage capacity of the transistor, this feature restricts improvementsin performance.

DESCRIPTION OF THE INVENTION

The invention described herein corresponds to a dc/ac inverter circuitspecially applicable as a conversion stage in photovoltaic installationsconnected to the grid, as shown in FIG. 3.

Said circuit minimizes EMC problems, and has a higher efficiency thanthose previously proposed.

The circuit of the invention is a single-phase inverter that isconnected to a direct current energy source and transforms it intoalternating current energy to be fed into an electric grid. The topologyof the inverter circuit essentially comprises:

-   -   a temporary energy accumulator that can consist of one or more        accumulators connected in series in one or more branches in        parallel with the energy source, across the direct current        connections of the circuit;    -   an inverter that is configured as a full bridge or H-bridge        comprising at least two parallel branches each with a pair of        switching elements in series, consisting of MOSFET, IGBT or        another type of transistor that adapts to this configuration        with or without their corresponding diodes in anti-parallel;    -   two auxiliary switching elements (T5D, T6D), with or without        their respective diodes in anti-parallel (D5D and D6D),        connected across the direct current connections and the input of        the H-bridge;    -   a branch with at least one auxiliary diode connected in        anti-parallel to the H-bridge across the respective connections        of the auxiliary switching elements and said H-bridge; and    -   at least two output points, which correspond with the        centre-taps of the branches of the H-bridge, which constitute        alternating current connections, to which inductances are        connected, it being possible to connect the electric grid        between the inductances.

The switching elements of the H-bridge, which comprise a first pair oftransistors (T1, T4) and a second pair (T2, T3), work as an inverterswitching at grid frequency and in synchronism therewith. During thepositive half-cycle T1 and T4 are on, while in the negative half-cycleT2 and T3 will be on.

The pair of auxiliary switching elements, T5D and T6D, are capable ofswitching synchronously by means of a given trigger signal or withindependent signals for each switch.

The operation of the converter can be explained during a switchingperiod of the positive grid half-cycle. T1 and T4 are on during theentire positive half-cycle. When T5D and T6D are on, the input voltageis applied across points A and B. The current flows through T5D, T1, T4and T6D.

When T5D and T6D are off, the current in the coils is closed throughDaux1, T1, T4 and Daux2. During this period of time a decoupling takesplace between the direct current side and the alternating current side.

The control signals are defined in a command unit that has at least onecomputation unit and software to implement the control strategy. Thecomputation module comprises at least one programmable electronicdevice, which can be a general-purpose microprocessor, amicro-controller, a digital signal microprocessor (DSP), anapplication-specific integrated circuit (ASIC), a field-programmablegate array (FPGA) or any combination of the foregoing, serving toestablish the updated values of the energy source working point.

The main advantages of the invention are:

-   -   The maximum ripple is half that in the H-bridge with PWM bipolar        modulation, which allows smaller output inductances to be used.    -   The problems associated with EMC are minimized.    -   Transistors T1, T2, T3, T4, which switch at low frequency, have        to be sized to withstand the input voltage, while the auxiliary        switching elements (T5D, T6D) which switch at high frequency,        have to be sized to support half the input voltage. This implies        an improvement when compared with the conversion structure that        EP1369985 proposes, wherein six transistors are also used but        all have to be sized to withstand the entire input voltage.    -   A further improvement with regard to the precedent EP1369985 is        that the T1-T4 and T2-T3 pairs switch at grid frequency, whereby        there are hardly any switching losses in them, it being possible        to use low saturation voltage semiconductors.

The number of semiconductors that conduct at all times in this converteris higher than in some of the conversion topologies of the current stateof the art, for which reason there will be more conduction losses.However, the switching losses in the circuit object of the invention aresmaller than in the other structures. This is because the semiconductorsthat switch at high frequency, T5D and T6D, in addition to switching athalf the input voltage, in contrast with the topologies already cited,this is also the maximum voltage that they have to withstand, thereforethis is the voltage for which they should be sized. Thus, a betterefficiency is achieved than in state of the art converters.

DESCRIPTION OF THE DRAWINGS

To supplement the description that is being made and with the object ofassisting in a better understanding of the characteristics of theinvention, in accordance with a preferred example of a practicalembodiment thereof, attached as an integral part of said description isa set of drawings wherein by way of illustration and not restrictively,the following has been represented:

FIG. 1.—Shows a configuration for a photovoltaic converter known in thestate of the art as an H-bridge.

FIG. 2—Shows another possible configuration for a direct voltage toalternating current or voltage converter, also applicable inphotovoltaic systems, according to an embodiment proposed in Europeanpatent application EP1369985 pertaining to the state of the art.

FIG. 3.—Shows a diagram of the structure of the circuit of the inventionaccording to a preferred embodiment.

FIG. 4.—Shows a diagram of the structure of the circuit of the inventionaccording to another preferred embodiment.

PREFERRED EMBODIMENT OF THE INVENTION

In the light of FIG. 3, a first practical embodiment of the inventioncan be described as a single-phase inverter circuit to condition anenergy source (8), formed by a photovoltaic array or another energysource adapted to supply a continuous input voltage (Vin) that can betransformed into an alternating grid voltage capable of delivering analternating current to an electric grid (9), comprising:

-   -   two direct current connections (6, 7) across which at least one        branch is connected with one or several temporary energy        accumulators;    -   an inverter with in an H-bridge configuration formed by at least        two parallel branches, a first pair of switching elements in        series (T1, T2) being connected to one branch and a second pair        of switching elements in series (T3, T4) to the other branch,        and it being possible for their respective diodes in        anti-parallel (D1, D2, D3, D4) to be connected in anti-parallel        to each switching element (T1, T2, T3. T4);    -   at least two alternating current connections (A, B)        corresponding to the centre-taps of the H-bridge branches, their        corresponding inductances (L1, L2) being connected to each one        of them, preferably with the same value L1=L2=L/2;    -   characterised in adding the following elements in its structure:    -   two branches connected in series with the H-bridge in the direct        current connections (6, 7) with two auxiliary switching elements        (T5D, T6D) with or without their respective protection diodes        (D5D, D6D) in anti-parallel; and    -   at least one of the branches with temporary energy accumulator        elements, connected across the points (6, 7) has a centre-tap        (12); and    -   a branch with two auxiliary diodes in series (Daux1, Daux2)        which are connected in anti-parallel to the input of the        H-bridge in some points (10) and (11). The centre-tap of this        branch is joined to the centre-tap of the temporary energy        accumulators (12). These branches signify a topological limit to        the voltage that can be applied to T5D and T6D, at the voltage        of C1 and C2 respectively.

A second alternative implementation of the invention is that presentedin FIG. 4 and which relates to a single-phase inverter circuit with asubstantially similar structure but which requires a smaller number ofsemiconductor elements. The distinguishing characteristics of thissecond configuration are:

-   -   the temporary energy accumulator (C) connected across the two        direct current connections (6, 7) does not need a centre-tap;    -   there is a single auxiliary diode (Daux) in the branch connected        across points (10) and (11).    -   In this case, the maximum voltage applied to the auxiliary        switches (T5D, T6D) is not limited by topology, but instead it        is conditioned by the switching characteristics of these        elements.

It is possible to add a command unit to either of these two structureswith a function similar to that of any of the control strategiespossible for known converters, which basically consists in governing theswitching of all the switching elements by means of a series of onsignals produced at the output of said command unit.

The terms in which this specification has been written are to be takenalways in the broadest sense and not restrictively.

1) A single-phase inverter circuit to condition and transform directcurrent electric power into alternating current electric power for theprovision thereof to an electric grid, comprising: two direct currentconnections across which at least one branch is connected with at leastone temporary energy accumulator; an H-bridge configuration invertercomprising at least two parallel branches, a first pair of switchingelements in series (T1, T2) being connected to one branch and a secondpair of switching elements in series (T3, T4) to the other branch; andat least two alternating current connections (A, B) corresponding to thecentre-taps of the H-bridge branches with an inductance (L1, L2) beingconnected to each branch; characterised in that at least one of thebranches with temporary energy accumulator elements, connected betweenthe two direct current connections, has a centre-tap and in that thecircuit additionally comprises: two auxiliary switching elements (T5D,T6D), connected across the direct current connections and the input ofthe H-bridge at some points; and a branch with two auxiliary diodes inseries (Daux, Daux1, Daux2) connected in anti-parallel at the inputpoints of the H-bridge, the centre-tap of which is joined to thecentre-tap of the temporary energy accumulators. 2) An inverter circuitaccording to claim 1, characterised in that each switching element (T1,T2, T3, T4) is connected in anti-parallel to a diode (D1, D2, D3, D4)respectively. 3) An inverter circuit according to claim 1, characterisedin that each auxiliary switching element (T5D, T6D) is connected inanti-parallel to a protection diode (D5D and D6D) respectively. 4) Aninverter circuit according to claim 1, characterised in that thetemporary energy accumulator elements are constituted by capacitiveelements, ultracapacitors, batteries, or combinations of these elements.5) An inverter circuit according to claim 1, characterised in that thetwo pairs of switching elements in series (T1, T2, T3, T4) of theH-bridge and the pair of auxiliary switching elements (T5D, T6D) aretransistors. 6) An inverter circuit according to claim 5, characterisedin that the type of transistors is selected between IGBT and MOSFET. 7)An inverter circuit according to claim 1, characterised in that it isconnected to a command unit adapted to govern the switching by means ofa series of on signals produced at the output thereof suitably aimed atthe switching elements (T1, T2, T3, T4) of the H-bridge and of the pairof auxiliary switching elements (T5D, T6D). 8) An inverter circuitaccording to claim 7, characterised in that the command unit contains atleast one computation module, which comprises at least one programmableelectronic device which is chosen from a general-purpose processor, amicrocontroller, a digital signal processor (DSP), anapplication-specific integrated circuit (ASIC) and a field programmablegate array (FPGA). 9) An inverter circuit according to claim 1,characterised in that the two pairs of switching elements in series(T1-T2, T3-T4) are capable of switching synchronously with the electricgrid by means of two complementary on signals. 10) An inverter circuitaccording to claim 1, characterised in that the two pairs of switchingelements in series (T1-T2, T3-T4) are capable of switching synchronouslywith the control signal that comes from the command unit and ispreviously calculated by the computation module. 11) An inverter circuitaccording to claim 1, characterised in that the pair of auxiliaryswitching elements (T5D, T6D) are capable of switching synchronously bymeans of a single on signal, defined by means of a known modulationtechnique, generated from the control signal that comes from the commandunit and is previously calculated by the computation module. 12) Aninverter circuit according to claim 1, characterised in that the pair ofauxiliary switching elements (T5D, T6D) are capable of switchingindividually by means of two on signals, defined by means of a knownmodulation technique, generated from the control signals that come fromthe command unit and are previously calculated by the computationmodule. 13) An inverter circuit according to claim 11, characterised inthat the on signals are defined by means of a pulse width modulation.14) An inverter circuit according to claim 1, characterised in that thevalues of the inductances (L1, L2) in series are the same. 15) Aninverter circuit according to claim 1, characterised in that it isincorporated in a converter of the transformerless type. 16) An invertercircuit according to claim 1, characterised in that an energy source isconnected to the direct current connections that is chosen from aphotovoltaic array and an electrochemical cell unit. 17) An invertercircuit according to claim 1, characterised in that it additionallycomprises at least one dc/dc converter, connected between the inputenergy source and the temporary energy accumulator. 18) An invertercircuit according to claim 17, characterised in that the converter isfitted with galvanic isolation between the facility and the grid,implemented by an output transformer.